Re: [RFC V1 00/16] arm64/mm: Enable 128 bit page table entries

[Anshuman Khandual]

On 08/04/26 5:43 PM, David Hildenbrand (Arm) wrote:
> On 4/8/26 12:53, Anshuman Khandual wrote:
>> On 07/04/26 8:14 PM, David Hildenbrand (Arm) wrote:
>>> On 2/24/26 06:11, Anshuman Khandual wrote:
>>>> FEAT_D128 is a new arm architecture feature adding support for VMSAv9-128
>>>> translation system. FEAT_D128 is an optional feature from ARMV9.3 onwards.
>>>> So with this feature arm64 platforms could have two different translation
>>>> systems, VMSAv8-64 and VMSAv9-128 could selectively be enabled.
>>>>
>>>> FEAT_D128 adds 128 bit page table entries, thus supporting larger physical
>>>> and virtual address range while also expanding available room for more MMU
>>>> management feature bits both for HW and SW. 
>>>>
>>>> This series has been split into two parts. Generic MM changes followed by
>>>> arm64 platform changes, finally enabling D128 with a new config ARM64_D128.
>>>>
>>>> READ_ONCE() on page table entries get routed via level specific pxdp_get()
>>>> helpers which platforms could then override when required. These accessors
>>>> on arm64 platform help in ensuring page table accesses are performed in an
>>>> atomic manner while reading 128 bit page table entries.
>>>>
>>>> All ARM64_VA_BITS and ARM64_PA_BITS combinations for all page sizes are now
>>>> supported both on D64 and D128 translation regimes. Although new 56 bits VA
>>>> space is not yet supported. Similarly FEAT_D128 skip level is not supported
>>>> currently.
>>>>
>>>> Basic page table geometry has been changed with D128 as there are now fewer
>>>> entries per level. Please refer to the following table for leaf entry sizes
>>>>
>>>>                     D64              D128
>>>> ------------------------------------------------
>>>> | PAGE_SIZE |   PMD  |  PUD  |   PMD  |   PUD  |
>>>> -----------------------------|-----------------|
>>>> |     4K    |    2M  |  1G   |    1M  |  256M  |
>>>> |    16K    |   32M  | 64G   |   16M  |   16G  |
>>>> |    64K    |  512M  |  4T   |  256M  |    1T  |
>>>> ------------------------------------------------
>>>>
>>>
>>> Interesting. That means user space will have it even harder to optimize
>>> for THP sizes.
>>>
>>> What's the effect on cont-pte? Do they still span the same number of
>>> entries and there is effectively no change?
>>
>> The numbers are the same for 4K base page size but will need
>> some changes for 16K and 64K base page sizes. Something that
>> git missed in this series, will fix it.
> 
> Oh, and it would be great to also clearly spell out the effect on
> hugetlb as well. I assume the available hugetlb sizes will change as well.

Sure will update the required information in the commit message as well as in
file arch/arm64/mm/hugetlb.c, where HugeTLB sizes support matrix is enlisted.